1. Field of the Invention
The present invention is directed to methods used in the fabrication of solid state devices and, more particularly, to methods of producing metal contacts.
2. Description of the Background
During the fabrication of solid state devices, it is necessary to use various types of interconnect structures such as contacts and vias. A via is an electrical connection between two metal layers and a contact is an electrical connection between anything other than two metal layers, such as between metal and silicon. Vias and contacts are used extensively in very large scale integrated (VLSI) circuits. An average VLSI circuit may contain 16 million vias and contacts.
It is desirable to completely fill vias and contact openings with metal or some other suitable electrically conductive material. However, when coating the surface of a substrate with a metal film by such processes as evaporation, ion beam deposition, or sputtering, self-shadowing may arise. Self-shadowing is a phenomena whereby the deposited material is deposited around the upper portion of a contact or via opening, restricting the diameter of the opening so that the bottom and lower portion are shadowed from the deposition process. Shadowing results in poor contact at the bottom of the opening, which results in high contact resistance, and the problems associated therewith, and can result in an open circuit. Another problem associated with shadowing is that with the contact opening pinched-off, debris, chemicals used in the fabrication process, or other foreign materials become trapped. When that occurs, the resulting contact has poor electrical properties and may ultimately fail. As circuit complexity rises, submicron contact openings result which have severe aspect ratios which exacerbate the aforementioned shadowing problems.
An attempt to solve the aforementioned problems is found in U.S. Pat. No. 4,874,493 entitled Method Of Deposition Of Metal Into Cavities On A Substrate. The method disclosed in that patent includes depositing a film of metal onto a flat surface and cavities in a substantially perpendicular direction to the surface. Simultaneously, the deposited film is resputtered from the flat surface by ion beam milling at an angle to the surface of the substrate for achieving the deposition of metal into the cavities and filling the cavities without leaving any film on the flat surface. A problem associated with the method of U.S. Pat. No. 4,874,493 is that processes such as ion beam milling and resputtering tend to be slow and inefficient. To completely fill a contact or via opening through resputtering or ion beam milling substantially reduces the throughput of equipment. At the same time, such processes require the use of larger amounts of metal or conductive material than are actually needed for filling the contact and via openings. Thus, the need exists for a method of preventing the aforementioned shadowing and debris trapping problems which is both an efficient use of equipment and material.